1. Field of the Invention
The present invention relates in general to servo control and in particular to servo control circuitry with voltage offset control and systems and methods using the same.
2. Description of the Related Art
Optical disks have been used for many years for the mass storage of digital data. Some well known examples of optical disks include digital audio compact disks (CD-DAs), compact disk read-only memories (CD-ROMs) and digital video disks (DVD-ROMs). Essentially, digital data is stored on a plastic disk with a reflective surface as a series of pits and land in the reflective surface. During playback, a beam of light is directed to the rotating reflective surface and the intensity of the photons reflected from the pits and land measured. A modulated electrical signal is generated that can be processed and the data stored on the disk recovered.
A basic configuration for the read (playback) mechanism has developed over a number of years. This configuration includes a pickup or sled, which is movable so that a laser, a lens, and array of photodiodes can be positioned directly over the data being read off of the disk. As the disk turns, the photons from the laser are reflected off the pits and land and received by the photodiodes which generate electrical signals having a current that is proportional to photon density.
The multiple signals output from the photodiodes represent both data detection and servo alignment information. The summation of the high speed data channel signal, which may be composed of the signals A+B+C+D from an astigmatic photodiode array, results in a composite signal with relevant data information between approximately 10 KHz and 60 MHz for current DVD players.
Servo information contained in these signals however, is at frequencies less than 1 MHz (for current spindle rotation rates of  less than 6000 RPM) down to DC. Because of these information rates, the data channel signal is sometimes AC-coupled to the data detection and summation circuitry mounted on an accompanying stationary circuit board. Otherwise, some degradation of the dynamic range must be accepted due to the dc content of the incoming signal.
The typical current signal generated by a photodiode is on the order of 1 uA. Transferring this signal directly down a flexible cable to the stationary circuit board therefore would seriously degrade the signal to noise ratio. Hence, transimpedance amplifiers, which convert the current from the photodiode array into a voltage for driving the cable, are mounted in the pickup to minimize noise and interference effects. The data detection, error correction, and servo systems are kept off of the pickup not only because the same interference reasons, but primarily to reduce the physical size and mass of the sled. These systems are mostly digital and switching noise on the pickup may degrade the signal to noise ratio.
On the fixed circuit board, the signals received from the pickup are amplified and converted to digital form. The primary processing functions, such as filtering, error correction and decoding then take place in the digital domain. At the backend, the processed digital data is reconverted to analog audio and video for driving the speakers and display screen.
One problem that can arise for the motors and actuators of the disk player when driving the servo control loops is voltage offset. For example, consider the pulse width modulated (PWM) signal that is used to drive the player spindle. This signal is generally a digital signal having a high voltage level approximating the supply voltage and a low voltage level approximating 0 volts or ground. The 50% voltage point between them is nominally the zero point for spindle motor rotation directional control. Typically, the PWM spindle control signal is generated off a clock signal having a 50% duty cycle.
Depending on the particular circuitry used, the precision of the PWM control signal can vary, which can introduce error in the spindle rotation. For example, the supply voltage may vary from chip to chip, may drift due to any one of a number of different operational factors, or may be subject to noise. Similarly, the low level voltage may not be fully clamped to ground. Moreover, the generating clock duty cycle will not be exactly 50% and the clock edges will have a finite rise and fall times. As a result, the high and low periods of the PWM signal may be slightly too high or slightly too low and thereby shorten or length the time the spindle motor is driven in a given direction.
In order to maintain the high quality playback for optical disk systems, a need has therefore arisen for methods and circuits for offset compensation in the servo control loops.
The principles of the present invention are disclosed with reference to an exemplary servo control circuit. A digital to analog converter converts digital servo control data into a pulse width modulated signal in response to a clock signal. A first conductor transmits the pulse width modulated signal to an external device while a second conductor transmits a reference signal derived from the clock signal to the external device. The external device differentially receives the pulse width modulated signal with respect to the reference signal.
The present inventive principles provide substantial advantages over the prior art. Among other things, circuits and methods are provided for accounting for drift in the supply voltage, a low voltage rail which is not sufficiently clamped to ground and noise. And moreover, errors in a servo control loop caused by the finite rise and fall times of the generating clock signals can also be canceled out using the reference signal.